Clipless pedals not engaging.
 

About 7 months ago I bought my first road clipless pedals (Shimano 105). I had no trouble at all clipping in and out during those 7 months. Recently I started developing knee pains, so I adjusted the cleat positioning on my shoes. Now I cannot, for the life of me, clip into the pedals. So far, I tried loosening the tension, putting grease on the cleats, and switching right and left cleats, all to no avail. I can't even clip in while standing on the pedals, pushing a fairly big gear up hill.
Anyone experience this/have a solution?
(Shimano 105 PD-5700)

Put the cleats back where they were and then only make small movements of the cleats. It sounds like you moved the cleats so far that they can't line up with the pedals.

I'd monkey with your cleat placement first. It sounds like you've done something, but I can't really imagine what that is with a surface-mount system like SPD-SL.

Can you clip the shoe in while you're off the bike and out of the shoes? That way you may be able to see what's happening.(BTW, I wouldn't advise doing this with an unmounted cleat, leave it on the shoe.)

Edit: I'm trying to imagine how this would happen. Did you move the cleat all the way "back" on the shoe? Possibly this would keep the back of the pedal from rotating up into the cleat, even with pressure applied?

It's possible that they're just worn out. They shouldn't be in 7 months, and I doubt that this is your current issue. But when the time comes, and you can't clip anymore, it's time to replace.

I have never been able to clip the shoe in by hand while off the bike; I always had to be on the bike pushing off in order to clip in. As for cleat positioning, I only moved the cleat towards the very front of the shoe, no lateral adjustments at all.

Even if you can't get it to engage that way, you should be able to see what the issue is. Try putting the tip of the cleat in and, while holding the shoe against the pedal, rotate the pedal and try to look from all angles. Also, cycle the retention mechanism on the pedal to make sure it is free.